In our copending United States patent application Ser. No. 851,042 filed Nov. 14, 1977, there is disclosed a multi-purpose portable hydraulic unit particularly useful for compressing heavy duty springs. Essentially there is provided a hydraulic piston rod extending from a hydraulic cylinder which will exert a downward pulling force on appropriate hooks engaging upper coils of a spring, the lower coils being held by a similar hook arrangement resting on a collar in turn seated on the cylinder head of the hydraulic unit. In this particular arrangement, the piston rod extends essentially coaxial with the helical turns of the spring.
There are many instances, however, where it is desired to compress a spring without having any of the compressing mechanism extending through the various coils. Such devices for accomplishing this end have taken the form of articulated frame means which can be scissored together to compress a spring. The scissoring together of the frame members is done in certain instances by a screw thread. In other instances, hydraulic power may be used.
While many of the available types of spring compressors are commercially successful, it would be desirable if such a compressor could be provided which not only avoided any mechanism within the helical turns of the spring but moreover assured a rectilinear motion in compressing the spring; that is, a compression force which acted in a direction exactly parallel to the axis of the coil so that all circumferential points of the spring are compressed uniformly. Further, such an accessory device should be capable of being operated by an appropriate hydraulic unit such as disclosed in our above referred-to copending application or alternatively by simple lead screw means.